1. Field of the Invention
The measurement of available thyroxine binding sites in serum is important in thyroid function screening tests, due to the innate relationship of the available thyroxine binding sites and the total thyroxine concentration to the concentration of free or unbound thyroxine. Thyroxine is normally almost completely bound in serum to carrier proteins. Roughly 70% is normally bound to TBG, 20% to thyroid binding prealbumin and 10% to albumin. The free thyroxine concentration in serum shows excellent correlation with, and is thought to be the true determinant of the thyroid status of the individual. The concentration of free thyroxine (T-4) in serum is very small and cannot be accurately or easily measured, the free T-4 being estimated as about 35 pg/ml. Therefore, one utilizes the relationship where free T-4 is proportional to the bound T-4 concentration divided by the free binding site concentration.
2. Brief Description of the Prior Art
Numerous techniques are presently available for determining thyroxine binding capacity. These techniques include Reverse-flow Electrophoresis, Robbins, Arch. Biochem. 73:461, 1956; Dextran-coated Charcoal, Roberts and Nikolai, Clin. Chem. 15:1132, 1969; Ion-exchange Resin, Roberts and Nikolai, Ibid.; Radioimmunoassay, Hamad, et al, J. Clin. Endocrinol. Metab. 31:166, 1970 and Competitive Ligand-binding Assay, Levy, et al, Ibid. 35:565, 1972.